1. Field of the Invention
The present invention relates generally to an electric power converter, such as an inverter unit, a power storage unit and so forth. More particularly, the invention relates to an electric power converter using an LC composite element reducing noise leaking from the electric power converter.
2. Description of the Related Art
An inverter unit has been widely used for operation of an alternating-current motor, such as an induction motor or the like. In the recent years, the inverter unit is also used as a controller for a power source of a vehicle so that a merit of variable speed operation by the inverter unit can be enjoyed satisfactorily.
In control of the conventional inverter unit shown in FIG. 16, PWM (pulse-width modulation) control systems have been widely used conventionally. The PWM control system includes a converter portion (forward converting portion) 2 constituted of a diode rectifier, a PWM control type inverter (reverse converting portion) 3, to which a direct current power output from the converter portion 2 is input, and a smoothing capacitor 4 connected in a direct current portion between the converter portion 2 and the inverter portion 3.
When an alternating-current power is input to the converter portion 2 from a commercial alternating-current source to be the power source, a direct-current power smoothed by the capacitor 4 is supplied to the inverter portion 3. Here, a semiconductor switching element 40 of the inverter portion 3 is PWM controlled to be converted into a predetermined voltage of the direct current power and an alternating current power of a predetermined frequency. As a result, a power of variable voltage and variable frequency is supplied to a load 6, such as induction motor or the like.
At this time, the semiconductor switching element 40 in the inverter portion 3 is controlled ON (conductive) and OFF(non-conductive) according to a PWM signal transmitted from a computer 28 via a driver circuit 43, and outputs a rectangular wave voltage and a load current to the load 6 to cause conduction loss determined by a conduction resistance of the semiconductor switching element 40 and the load current and a switching loss upon occurrence of transitional abnormality of voltage and current upon ON and OFF.
In the recent years, improvement of transient response of the semiconductor switching element 40 has been progressed in order to reduce the switching loss and IGBT (insulated gate bipolar transistor) having high speed switching characteristics has been developed to realize reduction of loss to contribute for down-sizing of the cooler of the device and down-sizing of the electric power converter represented by the inverter.
However, when transient voltage variation of the rectangular wave becomes acute, a current leaking from an earth capacitance 7 of the power cable connecting between an alternating-current motor as representative load 6 and an earth capacitance 7 of a winding of the alternating-current motor (hereinafter referred to as “leakage current”) is increased to elevate a peak value of the leakage current 8 in proportion to a rate of variation of the voltage relative to a time to cause high frequency resonation with parasitic inductance 50 of a power line. Also, the leakage current 8 may flows into the commercial alternating-current source 1 through the inverter portion 3 or the inverter 2 to affect to other equipments, such as to cause malfunction. Furthermore, the leakage current and an electromagnetic wave generated by wiring voltage of a leakage current path may penetrate into other equipments or to discharge radiation noise to television and/or radio antennas adjacent to the inverter.
As one example of prior art, the electric power converter employing the converter (forward converting portion) 2 constituted of the diode rectifier has been discussed hereabove. However, even with the electric power converter supplied a direct-current power output from a storage cell, such as a battery or the like, power of variable voltage and variable frequency can be supplied to the load 6, such as a power source of a vehicle, a cooling fan of a cooling device, a pump driving motor for circulating a cooling water, a hydraulic pump driving motor for hydraulic machine, a compressor driving motor for air conditioner and so forth by converting the direct-current power into the alternating-current power with a predetermined voltage and a predetermined frequency by PWM controlling the semiconductor switching element 40 of the inverter portion 3. Even in this case, radiation noise is generated.
Here, in order to preventing the leakage current 8 to be a cause of noise from flowing, a line filter 5 constituted of an X-connection capacitor 52 and a Y-connection capacitor 10 connecting a common mode transformer 9 as passive element connected to the power line shown in FIG. 16, the power line and the ground, is inserted in the input power line of the electric power converter in series with the load 6 or the power source for blocking flow of the high frequency leakage current 8 to the common mode transformation 9. Then, the blocked leakage current 8 flows to the Y-connection capacitor 10 and then fed back to the ground to significantly reduce the leakage current 8 flowing into the input power line, to avoid for other equipment to cause malfunction or the like, to reduce the leakage current 8 and electromagnetic wave generated by wiring voltage of the leakage current path so as not to penetrate into other equipment.
For down-sizing the line filter, there has been proposed the line filter 5 employing an LC composite element 15 disclosed in Japanese Patent Application Laid-Open No. 6-224045. As shown in FIG. 17, an anode 16 and a cathode of the capacitor are wound around a bar shaped magnetic body 19 of ferrite or the like in concentric manner to form the LC composite element 15 with the anode 16 and the cathode 17. The formed LC composite element 15 serves as the common mode transformer in the manner of winding. In order to form the X-connection capacitor 52 and the Y-connection capacitor 10 between the anode 16 and cathode 17, and the ground, third and fourth electrodes are wound together with the anode 16 and the cathode 17 to form the line filter 5 to realize compact and high performance line filter 5 for significantly reducing the leakage current 8 flowing in the input power line, for avoiding adverse effect, such as malfunction in other equipment, for reducing electromagnetic wave to be generated by the leakage current 8 and the wiring voltage of the leakage current path to avoid penetration into other equipment.
On the other hand, a method for damping the leakage current 8 by inserting a resistor 13 in the path to flow the leakage current 8, has been developed and reported in Institute of Electrical Engineers, 1995 Industrial Application Department National Convention No. 93 “High Frequency Leakage Current Restriction Effect and Designing Method using Common Mode Transformer”. In the method, the same phase winding to be zero-phase coil 26 is newly added to the common mode transformer 9 is inserted in the power line in series (hereinafter, the common mode transformer added the same phase winding will be referred to as CMT) for linking magnetic flux generated in a core portion of the common mode transformer 9 by the leakage current flowing through the power line to the newly added zero-phase coil 26 to short the output of the zero-phase coil 26 by a resistor 13. Thus, common mode current, namely the leakage current 8 flows through the resistor 13 for damping the leakage current 8 to significantly reduce the leakage current flowing through the power line to prevent adverse effect for other equipment, such as causing malfunction, to reduce the leakage current and electromagnetic wave generated by the wiring voltage of the leakage current path to avoid penetration into other equipment.
Conventionally, flow of the leakage current 8 to be a cause of noise is restricted using the line filter 5 constructed with the common mode transformer 9 connected to the power line, and the capacitors 10, 52 connected between the power line and the ground. In such case, the common mode transformer 9 to be used for the line filter 5 has to be provided a relatively large inductance characteristics in the extent of several mH. It is further required to accurately wind electrical conductor for a several turns on a magnetic core per each phase and an electrical conductor having relatively large diameter flowing the load current to the wound conductor. Therefore, size of the common mode transformer 9 becomes large and size of the line filter 5 becomes large. In case of the electric power converter having output of several kW, the line filter 5 becomes a size comparable with the electric power converter in such a manner that the line filter 5 has to be constructed in a casing separated from a casing of the electric power converter to encounter the problem of make the electric power converter bulky and high cost of the line filter 5.
Furthermore, in case of the line filter employing the conventional LC composite element 15, it is necessary to make the inductance value for shielding the leakage current 8 large for operation as passive circuit. To attain large inductance value, number of turns in winding the electrodes forming the common mode transformer 9 or, in the alternative, thickness of the magnetic body to be a core, such as ferrite is increased. In the former case, thickness is further increased for increased length of insulation paper. In either case, difficulty is encountered in down-sizing the line filter.
On the other hand, a circuit constants of the line filter 5 and CMT 11 are determined to achieve some reduction effect for the leakage current 8 within a range where installation condition of the electric power converter is determined by safety standard and so forth. Naturally, when one of installation conditions, such as a length of the power cable connecting the electric power converter and the load, characteristics of leakage capacity, is varied, it is possible that the desired reduction characteristics of leakage current 8 cannot be obtained. For instance, when the power cable is provided greater length than the predetermined value, it is possible that the desired reduction characteristics of leakage current 8 cannot be obtained. Particularly, in the line filter 5, when the characteristics of the semiconductor switching element 40 to be used in the electric power converter is varied or when the driver circuit 43 of the semiconductor switching element 40 is varied, a transient output voltage characteristics of the semiconductor switching element 40 becomes different to vary the waveform of the leakage current 8. Also, due to difference of frequency component to be contained in the leakage current, predetermined noise reduction characteristics cannot be obtained by the line filter 5.
Furthermore, in the technology disclosed in Japanese Patent Application Laid-open No. 2000-60407, the common mode current, namely the leakage current 8 can be damped by flowing the leakage current through the resistor 13 by inserting the CMT reported in Institute of Electrical Engineers, 1995 Industrial Application Department National Convention No. 93 “High Frequency Leakage Current Restriction Effect and Designing Method using Common Mode Transformer” between the converter portion as a power input stage of the electric power converter and the inverter portion to be an output stage for supplying the power to the load in parallel to the smoothing capacitor, and whereby can reduce the leakage current flowing through the power line significantly to reduce adverse effect for other equipment possibly cause malfunction. Also, the leakage current 8 and the electromagnetic wave to generate by the wiring voltage of the leakage current path can be reduced to avoid the problem of penetration into other equipment.